It is well known to use microwave apparatus for measuring the dielectric of materials which corresponds to the content of moisture or water within the material. For example, Scott and Yang, in U.S. Pat. Nos. 4,996,490 and 4,862,062, describe a microwave apparatus and method for measuring fluid mixtures. The Scott and Yang device and method utilizes a coaxial microwave transmission line, a free-running voltage controlled oscillator and a signal receiver for monitoring the change in frequency caused by impedance pulling of the oscillator due to the change in the dielectric constant of the mixture.
De, et al., in U.S. Pat. No. 4,902,961, describe a microwave system for monitoring the content of water in a petroleum pipe line. The De, et al. apparatus and method uses an S-band antenna and an X-band antenna to determine the complex dielectric constant of the fluid in the pipe line. The overall water content of the pipe line can be determined using an S-band main link that transmits a wave through a representative portion of the entire pipeline. Similarly, Swanson, in U.S. Pat. No. 4,812,739, describes an apparatus and method for using microwave radiation to measure the content of water in a fluid. The volume fraction of water in the fluid is measured by using first and second microwave beams having different frequencies. The beams are transmitted through the liquid and their respective absorption losses are calculated. The volume fraction of water is determined according to the absorption losses.
Several moisture measuring devices and methods are also known. For example, Knochel, in U.S. Pat. No. 4,546,311, describes a device for measuring moisture content. The Knochel patent uses microwaves produced by a transmitting arrangement formed by a signal source and antenna. The moisture laden substance is exposed to the field of radiation. An evaluation arrangement is connected to receive a phase change of the transmitted signal. The phase change represents the moisture content of the substance. Also, Read, in U.S. Pat. No. 3,500,182, describes an apparatus and method for the measurement of moisture in highly viscous pastes and similar materials. The Read patent measures moisture by passing high frequency electromagnetic signals through the viscous material. The viscous material is constrained in a chamber having a pair of opposed boundary plates extending edge-on in the direction of the movement of the material to form a combining guide path for the signals. The signals are evaluated before and after traversing the material to determine the moisture content in the material.
Bleackley, in U.S. Pat. No. 3,612,996, describes the measurement of the constituent proportions of a flowing substance using microwave energy. A method and apparatus are described having a main section wave guide, a branch section wave guide and a window. The three components make up a wave guide configuration which is a resonant microwave structure. The moisture in the material is determined by the resonant frequency of the structure. Also, Walker, in U.S. Pat. No. 3,818,333, describes a microwave window and antenna for measuring moisture of fluidized material. The described microwave windows extend parallel to each other and perpendicular to the axis of the microwave beam used. The antennas are in the form of dielectric rods. Ho, et al., in U.S. Pat. No. 4,423,623, describe a meter and method for measuring the composition and flow rate of a coal slurry and other similar mixtures. Signals are processed to determine a characteristic frequency of the waveguide or wavelength of the propagating microwave, which are related to the composition of the mixture within the waveguide. While Ho, et al., mention the cutoff characteristic of a waveguide, their teaching related to cutoff is misplaced. Ho, et al. teach spacing probes along the length of a pipe at even multiple of wavelength. It is clear from their teaching to space probes an even multiple of wavelength along the length of a pipe that they do not operate in the cutoff region. In the cutoff region, the wavelength is infinite and no propagation occurs. Ho's teaching concerning probe spacing is actually a consequence of standing waves existing in the pipe in the pass band region and is not related to the cutoff phenomenon. Further, Ho's interpretation of the use of the derivative function in determining a cutoff frequency characteristic is indecorous. The maximum of the derivative function produces a precise determination of cutoff and the value of the derivative maximum is inversely related to the conductivity of the process material. Ho, et al., on the other hand, teach that one or two minima of the derivative function should be used. The first minimum is well below the cutoff frequency and is of little value in the measurement. The second minimum is above the cutoff frequency and is influenced by errors typical of other pass band measurement techniques. Use of the minima does not give a precise measurement of cutoff frequency, nor does it yield the added benefit of a conductivity measurement.
Andresen, et al., in U.S. Pat. No. 5,124,653, disclose a method and apparatus for analysis of gaseous compounds, especially for determining the concentration of a gas in a gas mixture by microwave spectroscopy. Microwave pulses are used to excite rotational transitions of the molecules in the gas compound. Andresen's method relies upon the frequency response characteristics of the molecules of the individual components of a mixture.
Jakkula, in U.S. Pat. No. 4,755,743, describes a method and apparatus for measuring the moisture content or dry content of either high or low loss materials having a moisture content in excess of 50% utilizing a dielectric waveguide in contact with the material to be measured. An amplitude measurement is made upon a pass band signal that has been reflected at least ten times.
Because of the wide presence and usage of water in industrial processes, the measurement of water content or its complement, (i.e., percent solids in a water-based slurry), is the largest user need in the analytical or composition measurement field. Almost all raw materials and finished products contain water. Measurement of this moisture content is important for custody transfer.
In industrial processing, water is often used as a product carrier through the main processing steps and later is removed during the final manufacturing stages. Water measurement is important for controlling the mixing, mechanical dewatering and thermal dewatering stages. Mixing control has a high impact on product quality and raw material usage. Also, dewatering control strongly affects energy costs and custody transfer optimization. When the final product is still in slurry form, dewatering can also have a large impact on transportation costs.
Extremely large savings are possible on many processes from only a 1 or 2 percent improvement in moisture control. The cost of acquiring the moisture measurement is not a major expenditure if a needed improvement in measurement performance can be provided.
Despite the high user need for moisture measurement, most industrial applications do not have a satisfactory measurement solution. As a result, most of the time, either the measurement is not made or it is done manually by operators taking samples to a lab where they are weighed before and after drying to determine water content. The resulting measurement-time delays prevent control optimization and cause product waste and reduced throughput.
There are many different types of sensors available for making implied percent moisture or percent solids measurement. Examples of sensors include nuclear density, coriolis density, capacitance, microwave, infrared, conductivity, light refraction, consistence meter measurement of drag force and conveyor weighing systems. Most of the sensors only sense one variable and sometimes product temperature. As a result, they only work well on binary mixtures. An additional problem is that most industrial applications have other variables, such as conductivity, pressure, inhomogenious mixing, turbulence, etc., that cause major errors in the measurement. Also, many mixtures do not provide a large contrast between water and the other material, e.g., similar specific gravity or similar refractive index. Thus, the resolution is limited and small changes in the error variables cause big changes in the moisture readings when using sensors.
The factors discussed above make it very difficult for a user to put together a custom, multisensor system to compensate for a multivariable application unless measurement design engineers are hired and provided with a large budget. Because of very large economic impact, several major companies have independently undertaken such projects to obtain microwave oil/water cut measurement systems. Also for example, a large soap manufacturer has completed a project to obtain an infrared moisture measurement of soap powder. And, because the best combination of sensors depends on the myriad of different application conditions, measurement instrument vendors have only been able to justify the design of such multisensor systems where there is a very large sales potential on a fairly repeatable process such as paper machine optimization. As a result, there is still a large, unfulfilled market demand for a moisture measurement system that can provide sufficient long-term repeatability and resolution to permit switch over from manual, off-line analysis to automatic, on-line control.
There is thus a need for a unique apparatus and method for measuring any material parameter which may be inferred by measuring the electromagnetic properties of the material.
Recognizing the need for an improved apparatus and method for measuring moisture, it is, therefore, a general feature of the present invention to provide a unique electromagnetic sensing apparatus and method for the measurement of more electromagnetic variables for more complex mixtures with less effect from other potential error sources.
A feature of the present invention is to provide a unique electromagnetic measuring apparatus and method which can be used with a liquid, a slurry or a solid mixture flowing through a conduit.
Another feature of the present invention is to provide an electromagnetic meter and method for continuous on-line measurement.
Yet another feature of the present invention is to provide an electromagnetic moisture meter and method which has improved response time, especially with respect to off line measurement.
Yet still another feature of the present invention is to provide an electromagnetic meter and method which is used without contact to and is nonintrusive with, the material being evaluated.
Another feature of the present invention is to provide an electromagnetic meter and method for measuring moisture which has a low pressure drop associated with the measurement.
Yet still another feature of the present invention is to provide an electromagnetic meter and apparatus for measuring moisture which interrogates the entire cross-section of product for measurement of all the product.
Yet another feature of the present invention is to provide an electromagnetic meter and method for measuring moisture which is continuous and online.
Yet still another feature of the present invention is to provide an electromagnetic meter and method for measuring moisture which provides superior sensitivity and selectivity.
A feature of the present invention is to provide an apparatus and method for measuring percent solids which is effective in the measurement of entrained or dissolved solid materials within a fluid.
Yet another feature of the present invention is to provide an apparatus and method for controlling blending processes, for example, the mixture of gasoline with methanol or for putting specific amounts of ether into gasoline to increase the octane.
Yet still another feature of the present invention is to provide an apparatus and method for measuring conductivity using microwaves.
Yet still another feature of the present invention is to provide an apparatus and method for measuring compositional signatures of various compounds, such as for assisting in the determination of blends of hydrocarbons.
Yet still another feature of the present invention is to provide an apparatus and method for measuring steam quality, i.e., how much liquid is present in the steam.
Yet still another feature of the present invention is to provide an apparatus and method for measuring moisture in powders, such as, for example, pneumatic conveying applications, grains, plastic pellets, pulverized coal and the like.
Yet another feature of the present invention is to provide an apparatus and method for measuring the heat value associated with certain hydrocarbons, for example, methane, butane and the like.
Yet still another feature of the present invention is to provide an apparatus and method for evaluating polar molecular gases, which evaluation would provide a measure of concentration in various materials, for example, ammonia gas.
Yet still another feature of the present invention is to provide an apparatus and method for measuring the amount of magnetic material on a surface, such as, for example, recording tape.
Yet still another feature of the present invention is to provide an apparatus and method for determining the state of cure of synthetic rubbers.
Yet still another feature of the present invention is to provide an apparatus and method for measuring the amount of water vapor in a gas atmosphere, which provides a measure of humidity, dew point and wet bulb temperature.
Even yet still another feature of the present invention is to provide an apparatus and method for determining the condensation state of a gas.
Even still another feature of the present invention is to provide an apparatus and method for the evaluation of a batch interface measurement of different substances, for example, in a pipeline.
Yet still another feature of the present invention is to provide an apparatus and method for the measurement of the thickness of sheet material, such as, for example, paper products, plastics, fibers, cloth and the like.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.